I'm going to guess three shots at least...I've done this type of layering myself, though never to quite such good effect as here! ; Most likely, the main shot was the 10-second shutter exposure of the entire scene. ; Then, two secondary shots, each at a higher ISO yielding faster shutter speeds to freeze motion were taken - one for a Monorail arriving into the circle on the left, and another for a monorail leaving the circle on the right. ; Typically, the layers would be stacked with the main image over the two monorail shots. ; The two halves of the shots involving the monorails can each be kept, while the remainder of the layer can be erased - with one monorail stacked over the other and half a layer removed, you now have the two monorails just under the main globe layer. ; Using the eraser brush, you can just erase over the main layer (reduce opacity to help see) where the monorails are on the tracks, and it will show them through. ; Merge layers, and there you go.
I don't know if that's the technique used, but that's what I do. ; I like to use the technique when taking photos of a building where people are passing by on the sidewalk - if the parks are more crowded there's no way you'll get a hole in the crowd. ; So rather than wait for the whole frame to be devoid of people, just take a photo with noone in one half of your shot, then a second photo when noone's in the left. ; Or three shots if needed. ; When the shots are layered, you can erase away the people from the shot showing through the blank space in the background, and make the scene look empty.
Of course, best to use a tripod so the photos are perfectly aligned and at the identical focal distance.
Very cool shot indeed! ; And it takes some kind of patience to wait for the perfect moment for the monorails to come into frame.
